1. Field of the Invention
The invention relates to a device and method for video signal processing, and more particularly to a device and method for extracting a synchronization signal from a video signal.
2. Description of the Related Art
FIG. 1 shows a conventional circuit for extracting an SOG (Sync-on-Green) signal from a video signal. The circuit 1 comprises a clamp circuit 10 and a comparator 11. The clamp circuit 10 receives a video signal VS as shown in FIG. 2 and adjusts a direct-current level of the video signal VS. The comparator 11 compares the video signal VS and a fixed threshold THfix and generates an SOG signal SOG_OUT. However, since the video signal VS is compared with the fixed threshold THfix, the SOG signal SOG_OUT can not be extracted from the video signal VS precisely when the video signal VS is affected by noise, voltage shifting, or environment's change.
A method and device for dynamically adjusting an SOG signal of a video signal are disclosed in U.S. Patent Application Pub. No. US 2005/0270421. Referring to FIG. 3, a device for dynamically adjusting an SOG signal of a video signal comprises a clamp circuit 410, a comparator 430, a control unit 440, and a triggering level circuit 420. The clamp circuit 410 limits a voltage swing range of a video signal 405 so as to limit the video signal 405 within a clamp level and output a clamped signal 415. The triggering level circuit 420 determines a triggering level for the SOG signal so as to output a triggering signal 425. The comparator 430 compares the clamped signal 415 with the triggering signal 425 so as to output a compared signal 435. The control unit 440 receives the compared signal 435 and provides the triggering level circuit 420 with a feedback control signal 445 so as to adjust the triggering level (threshold) of the SOG signal. Meanwhile, the compared signal 435 is an output SOG signal. According to the device of FIG. 3, the triggering signal 425 with the adjusted triggering level is immediately used by the comparator 430 for comparing with the video signal VS. Thus, if the adjusted triggering level is not appropriate for the videos signal, an incorrect SOG signal is obtained and wrong images based on the incorrect SOG signal are immediately displayed.